Exhaust gas recirculation (EGR) systems recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and vehicle emissions by reducing throttling losses and combustion temperatures. In turbo-charged direct injection engines, a low-pressure EGR (LP-EGR) circuit may be implemented. The LP-EGR circuit recirculates exhaust gases from an exhaust passage downstream of a turbine to an intake passage upstream of a turbocharger compressor. In order to provide EGR over a wide-range of operating conditions, LP-EGR systems may utilize a flat EGR schedule wherein a fixed EGR percentage of fresh airflow may be maintained.
One example approach for providing desired EGR dilution is shown by Ma et. al in U.S. Pat. No. 6,014,959. Therein, a rigid connection is provided between an EGR throttle and a main air intake throttle, linking movement of the EGR throttle as a function of the movement of the main throttle such that EGR dilution is always in a fixed proportion to the intake air flow.
However the inventors herein have identified potential issues with such an approach. Delivering EGR as per Ma may lead to combustion instability and engine misfire events during transient operations due to excess EGR dilution. Particularly, in turbo-charged systems, providing EGR though the LP-EGR circuit may cause long transport delays, as the exhaust gases have to travel though the turbocharger compressor, high-pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. During selected transient operations, such as during a tip-out operation where the engine goes from a high load and high EGR rate condition to a low load and low EGR rate condition, EGR may need to be rapidly reduced. However EGR may not be purged from the intake manifold as rapidly as required. As a result, there may be elevated intake-air EGR dilution during the low load condition until the EGR is purged from the air intake system. The presence of increased intake-air dilution at low loads can increase combustion stability issues and the propensity for engine misfires.
In one example, some of the above issues can be at least partly addressed by a method for an engine comprising: in response to decreasing load while operating an engine with EGR, decreasing EGR, and fueling the engine with split fuel injection per cycle until EGR is less than a threshold. In this way, the EGR tolerance of the engine at low loads is improved.
As an example, an engine system may be configured with a low pressure EGR (LP-EGR) system to recirculate a portion of exhaust gas from an exhaust manifold, downstream of an exhaust turbine, to an intake manifold, upstream of an intake compressor via an EGR valve. The exhaust gas may be cooled upon passage through an EGR cooler before being delivered to the intake. Based on engine operating conditions, such as engine speed-load conditions, EGR delivery may be adjusted.
During a transition from operating the engine at a higher load to very low loads, such as during a tip-out operation, the throttle may be closed to reduce airflow and the EGR valve may also be correspondingly closed (or an opening reduced) to provide lower EGR at the lower loads. As such, at the low load conditions, engine dilution may not be required. However, the purging of the EGR may occur slower than desired due to transport delays in the LP-EGR system. The presence of more dilution than required can degrade combustion stability and induce misfires. In order to improve the low load engine combustion stability and EGR tolerance, while EGR is purged from the intake system, the engine may be transiently operated with split fuel injection. Specifically, until EGR reaches a desired rate (e.g., a zero EGR condition), fuel may be delivered as a first lean homogenous intake stroke injection and a second locally rich stratified compression stroke injection, while maintaining overall combustion air-fuel ratio at stoichiometry. Then, when the EGR has been purged and the desired EGR rate reached, single fuel injection may be resumed.
In this way, by temporarily performing a split fuel injection when operating the engine at low loads in the presence of more EGR dilution than required, the EGR tolerance of the engine may be improved. By delivering a larger portion of the fuel during the intake stroke and the remaining fuel during the compression stroke around the spark event, a rich air-fuel mixture may be created around the spark plug which expedites the burn time of the mixture. In addition, the split injection enables a later spark timing to be utilized in which further improves combustion stability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.